For various location based services as well as for handling emergency situations, it is essential to determine as accurately as possible the geographical location of a user of a mobile communication terminal. Mobile communication terminals include, for example, mobile radio (cellular) telephones or personal digital assistants (PDA) as well as other portable computers with communication modules for mobile radio networks, such as GSM (Global System for Mobile Communication) or UMTS (Universal Mobile Telecommunication System). Although there are mobile communication terminals available which include a GPS-receiver (Global Positioning System) or another satellite-based positioning system, there is still a need for other location methods, as for example locating mobile communication terminals which are not equipped with such positioning systems, mobile communication terminals which have their GPS turned off or inside of buildings where the GPS signal is too weak. It is known from the mobile network which antenna the user is using. However, particularly in rural areas, an area served by an antenna can cover a very large geographical area. Unfortunately today calculations of these areas are not accurate, often they are too large or they are not reliable, and in reality mobile communication terminals are often located outside of these areas (low hit rate).
It is essential to determine smaller and more reliable location areas of mobile users in geographical areas covered by mobile radio networks. Particularly, there are legal regulations which require the geographical locations of users of mobile communication terminals to be provided as shapes which define a geographical area where the mobile user is most likely located. These shapes are required to be described in a simple and straightforward geometric fashion, e.g. in the form of polygons, a circle or one or more ellipses.
GB 2352134 describes a method of locating a mobile telephone based on a calculation of expected signal properties such as signal strength or observed time differences for a plurality of possible locations, e.g. arranged in a grid. The expected signal property is compared to a measured signal property. Based on the comparison, determined is the probability that the mobile telephone is at one or more of the locations. Thus, the method of GB 2352134 is based on the actual values of the field strength or time differences measured at the mobile telephone. However, these values would have to be transmitted from the mobile telephones to a centralized measuring system and are therefore not necessarily available for locating a mobile telephone. Furthermore, manufacturers of proprietary network components do not necessarily make such values available to the operators of mobile networks or they sell them at considerable cost. The method does also fail when there are less than 3 antennas available, or if the visible antennas are arranged along a line, for example, in mountain areas.
Patent application WO 99/07177 describes a method of determining the location of a mobile communication terminal using elliptical position estimates as an improvement over circular estimates. However, the teachings of WO 99/07177 are limited to defining an elliptical position estimate based on 1000 simulated locations which form a substantially symmetrical and elliptical statistical sample. Typically, however, measurement data from real networks do not provide symmetrical and elliptical clusters of possible locations of a mobile user. The location areas do have in reality very different shapes, depending for example on the position of other antennas or the elevation model of the geographical area, for example hills and valleys. Furthermore, when polygons or ellipses are created around estimated locations, they can cover very large areas, because just one single estimated location point, far away from all other location points, can possibly enlarge the polygons or ellipses more than 100 times. Thus, for real life scenarios it is necessary to reduce the hit rate and therefore find polygons or ellipses which cover much smaller location areas, in fact as small as possible, but still with a satisfactory hit rate (probability) of over 95%, for example.